Excitatory amino acid neurotransmitter systems are important in memory and learning (Izumi et al.; Morris et al.). A specific set of excitatory receptors of particular interest are those that selectively bind N-methyl-D-aspartic acid (NMDA). Activation of this transmitter system is probably a necessary step in the early formation of certain types of memory (Collingridge et al.), yet over-stimulation of these receptors can be toxic and result in cell death (Rondouin et al.). There is an increasing body of evidence that excitatory amino acid neurotransmitter systems are progressively affected in the course of Alzheimer's disease (Greenamyre, et al.; Foster et al.; Penney et al.), and have been implicated in a number of central nervous system (CNS) disorders including epilepsy (Wong et al.), hypoxia/ischemia brain damage (Rothman), Huntington's disease (Young et al.), AIDS encephalopathy (Giulian et al.), and amyotrophic lateral sclerosis (Spencer et al.).
Epilepsy, for example, is a complex disease process which is still poorly understood. A cellular or biochemical etiology cannot usually be identified. Treatment is generally focused on seizure control, with hopes that the condition will ameliorate on its own. Unfortunately, drug therapy for seizures is subject to much individual variation. Thus, treatment is frequently by trial and error. Various combinations of drugs in various dosages are typically administered in an effort to provide some degree of seizure control without causing overt neurotoxicity. Unfortunately, some 20-40% of epileptic patients fail to experience satisfactory results with the drugs currently available (Goehring et al., 1990). Moreover, many of these drugs produce troublesome side effects when administered over the long time periods required in treatment, which frequently spans years or decades. The design of drugs which act only at sites of inappropriate neuronal excitation should minimize unfavorable side effects such as ataxia and sedation. To this end, we have sought to develop compounds that will selectively attenuate activity at specific subsets of excitatory neuroreceptors.
The NMDA-sensitive receptor sites comprise a subset of the excitatory neuroreceptors that are activated by L-glutamic acid. The NMDA neuroreceptors are linked to non-selective ion channels (Nowak et al., 1984), which are voltage dependent and permeable to calcium. The receptor complex also has a strychnine-insensitive binding site for glycine. For channel opening to occur, apparently both glutamate and glycine binding sites must be occupied (Johnson and Ascher, 1987). Consequently, antagonism of glycine binding inhibits NMDA responses (Kemp et al., 1988). Since glutamate functions as an excitatory neurotransmitter, it is not surprising that inhibitors of NMDA activity have anticonvulsant properties (Dingledine et al., 1990). The quinoline derivative 7-chlorokynurenic acid is a competitive inhibitor of glycine binding at the NMDA receptor, and this is thought to be the mechanism of its anticonvulsant activity (Wong et al., 1986; Singh et al., 1990).
There is evidence that NMDA receptors are responsible, at least in part, for the neurotoxicity seen in ischemia, and to excitatoxic cell death (Rondouin et al.). Excitatoxicity is the neuronal degeneration caused by exposure of central nervous system tissue to excitatory amino acids. It has been shown that non-competitive antagonists of NMDA receptors protect cortical and hippocampal cell cultures against glutamate neurotoxicity (Rondouin et al.). Calcium entry through the NMDA receptor channel is thought to be the mechanism by which glutamate released from nerve terminals can regulate long-term physiological events and, under pathological conditions, precipitate neurodegeneration. In addition, it has been recently reported that the glycine/NMDA receptor is involved in the behavioral effects of cocaine (Morrow et al., 1995) and in ethanol activity in the CNS (Morris and Leslie, 1996). NMDA receptor antagonists have also been shown to possess analgesic (Dickenson and Aydar, 1991), anti-depressant (Trullas and Skolnick, 1990), and anxiolytic effects (Kehne et al., 1991). Compounds of the present invention may accordingly be useful in the management of pain, depression, and anxiety, and in the prevention or reduction of dependence on addictive agents such as narcotics.
It should be feasible to design and synthesize efficacious amino acid analogs as therapeutic agents for the NMDA receptor complex once binding characteristics are understood. However, it needs to be remembered that a simple agonist for the NMDA receptor could be neurotoxic, while an antagonist might potentiate memory loss. Research relating to the present application has been directed towards the glycine modulatory site on the NMDA receptor complex (Johnson et al.). Development of a strategy for pharmacological intervention at this site should lead to the production of drugs with mixed agonist-antagonist activities that could be clinically useful while having minimal side effects. Although potent glycine site ligands have been synthesized, in most cases these have had poor in vivo anticonvulsant activity (Rowley et al., 1993; Leeson et al., 1992). We have reported previously on several kynurenic acid derivatives that had anticonvulsant activity (Nichols and Yielding, 1993). These compounds also compete for .sup.3 H!-glycine binding in synaptosomal assays. Leeson et al. (1992) showed that in another series of rigid ring glycine ligands, the 2-carboxytetrahydroquinolines, 4-urea derivatives were more potent glycine antagonist than either 4-amines or 4-amides. However, the 2-carboxytetrahydroquinoline derivatives had poor in vivo activity following intraperitoneal (ip) or intravenous (iv) administration (Leeson and Iversen, 1994).
In an effort to produce anticonvulsants with in vivo potency, we have synthesized a unique series of 4-urea-5,7-dichlorokynurenic acid derivatives. These were evaluated for anticonvulsant activity in maximal electroshock (MES), subcutaneous pentylenetetrazole (Met), and threshold tonic extension (TTE) tests. They were also assayed for strychnine-insensitive glycine binding inhibition using a synaptosomal membrane preparation. One compound was also tested for phencyclidine (PCP) activity at the NMDA receptor, and for GABA-A effects.